1. Field of the Invention
The present invention relates to a variable reluctance sensor for sensing moving ferrous targets and, more particularly, to a variable reluctance sensor for use in applications where space is relatively limited which includes a coil of wire, one or more pole pieces or stators and an injection molded permanent magnet formed as a bobbin for carrying the coil of wire which enables the sensor to generate a relatively higher output signal level than known sensors in a relatively limited amount of space.
2. Description of the Prior Art
Variable reluctance sensors (also known as "magnetic pickups") are known in the art for sensing both linear and rotary motion. Examples of various variable reluctance sensors are disclosed in U.S. Pat. Nos. 2,304,866; 3,469,662; 3,719,842; 3,721,968; 3,722,549; 3,947,711; 3,961,214; 4,161,665; 4,471,652; 4,626,781; 4,558,158; 4,700,133; 4,721,864; 4,772,815; 5,023,546; and 5,023,547. Such sensors are normally self-powered and generally include a permanent magnet, a coil of wire wound about a bobbin and one or more pole pieces. In operation, movement of a ferrous target relative to the sensor causes a voltage to be induced at the output terminals of the coil of wire that is proportional to the time rate of change of magnetic flux lines cutting the coil. The time rate of change of magnetic flux lines cutting the coil varies as a function of the reluctance of the air gap which, in turn, varies as a function of the distance of the magnetic target relative to the pole pieces.
Such sensors have been known to be used to sense the wheel speed of vehicles equipped with anti-lock brake systems (ABS), for example, as disclosed in U.S. Pat. Nos. 3,469,662; 3,719,841; 3,772,549; 5,023,546 and; 5,023,547. In such applications, a sensor target is normally configured as a toothed wheel and mechanically coupled to rotate with one of the vehicle wheels, while the sensor is normally mounted to the wheel hub adjacent the target. With such a configuration, rotation of wheel, and, in turn, the toothed wheel, results in output pulses from the coil having a frequency proportional to the speed of the vehicle wheel.
There are several problems with such variable reluctance sensors. For example, some known variable reluctance sensors are relatively expensive to manufacture. In addition, applications where the sensor is used to sense the speed of a vehicle wheel, the sensor is physically attached to the wheel hub, for example, as described in U.S. Pat. No. 5,010,290, which results in other known problems. For example, in such applications, the sensor can be relatively difficult to install to a wheel hub by an automated manufacturing process. In addition and probably most important is that the available space for the sensor in such an application is relatively limited. The physical space limitation, in turn, limits the size of the magnet and, in turn, the strength (e.g. gauss output) of the magnet. Since the output voltage level of the sensor is a function of the magnet strength, such variable reluctance sensors have been known to produce relatively low output signal levels. Because of the electrical noise level in certain applications, such low output signal levels are undesirable.